The present invention relates to a lens sheet such as a prism sheet used for enhancing luminance in normal direction in a back light system, etc. as an illuminating planar light source device of liquid crystal display apparatus, etc., or a double-sided lenticular lens sheet used in a projection screen as a display panel of projection television, microfilm reader, etc., and a method of manufacturing the same, and particularly to a lens sheet having no optical defect such as spot pattern, glare or the like and a method of manufacturing such a lens sheet.
Recently, in an article which is driven by battery, such as portable personal computer having color liquid crystal display apparatus, portable liquid crystal display apparatus and video-integrated liquid crystal display television, etc. having color liquid crystal display panel, or the like, it has been an obstacle to the prolongation of operation time duration by a charged battery that the power consumption of the liquid crystal display apparatus is significant. Especially, a rate of the power consumption of the backlight system used in the liquid crystal displays apparatus to the total power consumption of the liquid crystal display apparatus is great. Therefore, in order to prolong the operation time duration by the charged battery so as to enhance the utilization of the article, it is important to reduce the power consumption of the backlight system as much as possible. However, it is not preferable to lower the luminance of the backlight system by suppressing the power consumption of the backlight system, because it becomes hard to observe the liquid crystal display. In JP-3-69184(U), etc., there is disclosed a backlight system provided with a lens sheet having a plurality of lens units such as elongated prisms on the surface thereof and disposed at the light emitting side of the light guide to thereby improve optical efficiency, so that the power consumption of the backlight system is reduced without lowering the luminance.
As the lens sheet mentioned in the above, there is proposed a lens sheet having the lens portion made of activation energy curing composition such as ultraviolet curing composition which has good transferring property and good yieldability. In such a lens sheet, for example, the lens portion of cured activation energy curing composition is integrally formed with a transparent substrate such as transparent resin film or transparent resin sheet.
In such a lens sheet having the lens portion which is made of activation energy curing resin, there is the following problem: In case where the lens sheet is used as a component of the backlight system, a spot pattern which is considered to be performed on the basis of the deformation of the lens shape caused by the shrinkage of the activation energy curing resin in polymerization when the activation energy curing resin is cured and molded. The spot pattern becomes an optical defect to deteriorate the optical property of the backlight system.
On the other hand, in the projection screen of a projection television, microfilm reader, etc., there is used a lenticular lens sheet having lenticular lenses formed on both sides in order to form a good projection image. Conventionally, such a lenticular lens sheet has been manufactured by injection molding method in which transparent resin material is used, or press molding method in which a lens mold having a lenticular lens transferring pattern is pressed to the resin plate under heating to thereby transfer the pattern onto the surface of the resin plate. The transparent resin material is, for example, acrylic resin, polycarbonate resin, vinyl chroride resin, styrene resin, etc.
However, in case of the injection molding method, it is difficult to manufacture a lenticular lens sheet of large size, and therefore this method is restricted to obtain the lens sheet of relatively small size. In case of the press molding method, since a long time is required for heating/cooling cycle of the resin plate and lens forming mold, many lens forming molds are necessary for mass production of the lens sheet, and therefore enormous expense is necessitated for providing the manufacturing apparatuses in manufacturing the lenticular lens sheet of large size.
There has been proposed a manufacturing method of the lens sheet in which an activation energy curing composition is poured into the plate-shaped lens forming mold, then irradiated with the activation energy to cure the composition to perform the forming. This method makes it possible to reduce the forming time to thereby enhance the yieldability. However, there is a problem in the method that bubbles are involved in the resin when pouring the resin into the lens forming mold. In order to avoid such a problem, it is necessary to perform the deaeration in the additional step, or to pour the resin into the mold slowly. Therefore, it is insufficient to take the above conventional method for mass production. Especially, in the method where the lens forming mold having specific transferring pattern is used, the air tends to be retained in the groove of the mold, thereby occurring the air bubbles in the resulting lens sheet. The air bubbles formed once is not easily removed and causes a generation of defect of air bubble in the lens.
In order to prevent such a bubble generation, there is proposed in JP-1-192529(A) a method in which the ultraviolet curing composition is supplied on the lens forming mold so as to form a composition pool, a base film is put on the pool, the pool is averaged in height by pressing with roller on the lens forming mold via the base film to fix the base film to the composition, then the composition is irradiated with the ultraviolet light to perform the curing and molding of the composition, and then the mold is removed to obtain the lens sheet.
However, according to such a method, it is difficult to obtain a lens sheet having uniform thickness, and thus the lens sheet tends to have lower uniformity in thickness, resuting in the problem of causing the deterioration of the lens performance such as distortion of the image, etc. Additionally, in this method, since the shrinkage of the activation energy curing composition in polymerization is considerable when curing, the lens shape is not sufficiently transferred so that the lens sheet obtained is deviated considerably from the shape designed, or the strain remains in the lens sheet. A gap tends to occur between the lens portion and the transparent substrate of sheet to cause delamination. Especially in the double-sided lenticular lens sheet, the lower uniformity in thickness and lower coincidence in the optical axes of lenticular lenses at both sides deteriorate the screen performance, and therefore it is necessary to control the thickness of lens and lens coincidence at both sides.
Recently, it has been required super fine image displaying, and in order to comply with such a requirement, there is proposed a method of continuously forming a lenticular lens made of ultraviolet curing resin on both sides of the transparent substrate with use of the cylindrical lens-forming mold in JP-1-159627(A) or JP-3-64701(A).
In case of such a method where the double-sided lenticular lens sheet is manufactured with use of the ultraviolet curing composition, lens shape is slightly deformed on the basis of the polymerization shrinkage of the ultraviolet curing composition when the ultraviolet composition is polymerized and cured. Therefore, in a screen provided with the double-sided lenticular lens thus obtained, there occurs a glare which is considered to be caused by lower uniformity in direction of the reflected light of the external light. The glare is one of the optical defects which deteriorate the optical performance of the screen, etc.
Therefore, an object of the present invention is to provide a lens sheet having no optical defect such as spot pattern, glare or the like and a method of manufacturing such a lens sheet by suppressing ocurrig of deformation of the lens shape caused by polymerization shrinkage of the activation energy curing composition.
A lens sheet according to the present invention comprises a transparent substrate; a lens portion made of activation energy curing resin having a plurality of lens units, the lens portion being disposed on at least one side of the transparent substrate; and a relaxation layer disposed between the transparent substrate and the lens portion, wherein a thickness of the relaxation layer is within a range of 1% to 30% of a height of the lens units.
A method of manufacturing a lens sheet according to the present invention comprises the steps of:
providing an activation energy curing composition to an area between a lens portion transferring pattern surface of a lens forming mold and one side (inner surface) of a transparent substrate to form a composition layer between the lens forming mold and the transparent substrate;
making a thickness of the composition layer uniform with use of a nip roll disposed so as to confront the other side (outer surface) of the transparent substrate;
irradiating the composition layer with activation energy via the transparent substrate to cure the composition layer and form a lens portion made of activation energy curing resin; and
releasing the lens portion and the transparent substrate as an integrated form from the mold,
wherein a relaxation layer made of activation energy curing resin is formed between the lens portion and the transparent substrate on the basis that nip pressure of the nip roll is regulated by pressure regulating means.
According to the present invention, since the thickness of the activation energy curing resin composition layer is made uniform with use of the nip roll under regulated nip pressure by pressure regulating means while the lens portion is formed via the relaxation layer having a specific thickness, so that the deformation of shape of the lens portion caused by the polymerization shrinkage of the activation energy curing resin composition is moderated by the relaxation layer formed simultaneously, there is provided a lens sheet such as prism sheet or double-sided lenticular lens sheet, etc. having no optical defect such as spot pattern, glare or the like.